Polymer beads are usually produced by free radical suspension polymerization, a well-known process developed in 1909 [F. Hofmann and K. Delbruck, Ger. Pat. 250,690 (1909)]. The most important use of porous beads based on poly[styrene-co-divinylbenzene]is in the synthesis of ion-exchange resins. Significant specialty materials markets exist in areas related to separation science (e.g. water purification, high-performance liquid chromatography, etc.).
While a suspension polymerization technique is simple, the beads that are currently obtained therefrom lack size uniformity, i.e. they generally have a broad particle size distribution. However, for some applications (e.g. chromatography, calibrations, diagnostics) beads of uniform size are required. Uniform size beads can be obtained by size classification of the product obtained by a suspension polymerization, but this is tedious and only provides a low yield of the most useful fractions. Japanese Kokai No. Sho. 53-86802 describes a typical suspension polymerization technique.
To circumvent this problem, i.e. to eliminate the handling of large quantities of waste beads and the size classification process, uniform beads ranging in size from about 1-100 .mu.m have been produced by multi-step swelling polymerizations according to the disclosures of, c.f. Ugelstad (U.S. Pat. Nos. 4,186,120 and 4,336,173) and Hattori et al. (Japanese Kokai Tokyo Koho JP 61-190,504; 61-215,602; 61-215,603; 61-215,604; 61-215,605; 61-231,043; 61-283,607). The principle of such preparations is simple and entails performing at least two separate polymerization steps with first being an emulsifier-free emulsion polymerization to provide monodispersed "seeds" having uniform size, typically near 1 .mu.m [Goodwin J.W. et al., Brit. Polym. J., 5, 347 (1973)]. The seed particles are capable of absorbing about 0.5 to 30 times their own volume of a low molecular weight compound such as a solvent or a monomer. When the compound is a monomer and the absorption step is followed by a second polymerization, the process is called a "seeded polymerization". The degree of enlargement of the seeds is proportional to their swelling ability; this has not been found sufficient to produce beads with a diameter of at least 5 .mu.m in one step.
A seeded polymerization is the easiest way to produce a product in which the size of the secondary beads exceeds that of the primary particles by factor of about two or less [Vanderhoff J.W., et al.; J. Dispersion Sci. Technol., 5, 231, (1984)]. In this technique, the primary particles are swollen with a highly dispersed monomer-initiator mixture that is emulsion stabilized with a surfactant. The second polymerization of the swollen primary particles is then started by simply raising the temperature. The swelling process (involving only one mixture) has the advantage of simplicity, but has been limited in its ability to increase the size of the primary particle. Also, the beads produced have been non-porous.
Theoretical considerations [Ugelstad J. et al., Makromol. Chem, 180, 737 (1979)]based upon extended Flory-Huggins theory predict that a particle containing a substantial amount of an oligomer or an organic solvent which is substantially water-insoluble should exhibit an enormous increase in its swelling capacity by a monomer. The volume of the swollen particle may exceed the volume of the initial particle by up to 1,000 times. Subsequent polymerization of the monomer which has been absorbed into the swollen particle should result in the formation of a highly enlarged polymer particle that retains its original shape. In his two-step swelling process, Ugelstad also uses an approach similar to the traditional method of production of macroporous beads, i.e. one part of the liquid transferred into the primary seed during the second swelling step is a porogenic solvent. Due to the fact that the volume of the swollen particle is up to a few orders of magnitude larger than that of seeds, the concentration of the polymer originating from the primary particle in the swollen particle is then very low (less than 2%).
Another synthetic approach to uniform particles based upon a dispersion polymerization is described in U.S. Pat. No. 4,524,109. Its principle is simple. A solution of monomer in a solvent which dissolves the monomer but does not dissolve the polymer from the monomer is polymerized using a dissolved free radical initiator. During dispersion polymerization polymer chains grow in the solution until they become large. Then they start to precipitate. To prevent the precipitated polymer from aggregating into a formless mass, the polymerization mixture also contains a dissolved steric stabilizer and surfactant. The resulting particles retain uniform spherical shape and size up to about 10 .mu.m. A serious drawback to the dispersion polymerization is that it is restricted to only monovinylic monomers, thereby prohibiting the synthesis of uniformly sized crosslinked polymers.
Almost uniform beads sized up to about 1 mm have also been produced by more unusual techniques. For example, a method is described by Rhim et al. [NASA Tech. Briefs, September 1989, p. 98]in which the organic part of the polymerization mixture containing monomers, an initiator, and optionally other compounds is continuously injected through a vibrating capillary onto the surface of liquid nitrogen and then polymerized by irradiation. The method results in beads with sizes that exceed the optimum for analytical chromatographic applications.
While there is no unique definition of macroporosity in the literature, it is generally accepted and will be for the purposes of this application that the main feature of a macroporous polymer is a solvent regain (ability to accommodate a nonsolvating solvent within the pores, which relates to the porosity) of greater than 0.1 ml/g, preferably greater than 0.5 ml/g. While some authors also quote data on minimum pore size (&gt;5 nm) and specific surface area in the dry state (&gt;5 m.sup.2 /g), these are only additive to the solvent regain property.
A copolymerization of mono- and di-vinyl compounds leading to a macroporous polymer is a special type of heterogeneous crosslinking polymerization. The reaction mixture will contain not only the monomers but also an inert solvent which will act as a porogen. After the polymerization starts in many sites at once, the polymerizing radicals add both the monovinylic monomer and crosslinking polyfunctional monomer. The pendant double bonds of the crosslinking polyfunctional monomer frequently react with another double bond of the same polymer chain rather than with that of another chain because of the relatively high dilution of the system (which means that the other polymer molecules are sometimes not available in the close vicinity of the reaction site). Thus, intramolecular crosslinking is more prevalent than is the case for the preparation of conventional gel polymers. When a thermodynamically good solvent is used as a porogen (e.g. toluene for styrene-divinylbenzene), the original nuclei are swollen by solvent and monomers which continue to polymerize and the crosslinking density continue to increase until it approaches the point where to polymer, even in the swollen state, cannot occupy all available space. As the polymerization proceeds further the chunks of primary gel shrink more and more, exuding the inert solvent component. The decreasing solvation results in separation of a polymer rich phase. This phase is still swollen and behaves almost as a liquid. Interfacial tension forces it to achieve the energetically most preferred shape, i.e. a sphere. This means that during a suspension polymerization a single observable droplet in fact consists of a very large number of spherical gel nuclei swollen with both the monomers and solvent, but separated by a liquid phase having a similar composition. These gel nuclei grow as the polymerization proceeds until they eventually touch each other. Because interpenetration of cross-linked molecules is not feasible, the entities (then called globuli) retain their identity until the end of the polymerization. Some macromolecules in the late state of polymerization can even grow through more than one microgel nucleus thus connecting them together. Also, when the crosslinking monomers, e.g. divinylbenzene, is more reactive than the monovinylic monomer, e.g. styrene, it polymerizes more rapidly and a significant part of the divinylbenzene is consumed at the initial stage of the polymerization. The remaining monomers are predominantly monovinylic and thus the globuli are less crosslinked at the outside in comparison to the inside.
Low molecular weight porogenic compounds produce macroporous polymers with high specific surface areas (up to &gt;700 m.sup.2 /g), but the pore size is very small. Two methods are available to obtain macroporous products with large pores. The first entails using a large excess of non-solvent diluent as a porogen during the synthesis, while the second involves the use of polymers or polymer solutions as porogens.
Attempts have been made to use soluble polymers as porogenic agents in essentially the same manner as the thermodynamically poor solvents [Abrams J., Ind Eng. Chem., 48, 1469 (1956)]. For example, Japanese Kokai No. 53,086,802 discloses a traditional aqueous suspension polymerization of styrene, ethylstyrene, and divinylbenzene in the presence of a large amount (40-60 vol. %) of an organic solvent and a small amount (&lt;8 vol. %) of a linear polystyrene polymer. Also, soluble polystyrene acting as a porogen in the copolymerization of styrene with more than 10% divinylbenzene only when its molecular weight exceeds 50,000 and its weight fraction in the mixture was more than 10% is described by Seidl et al. [Adv. Polym. Sci., 5, 113 (1967)]. When the amount of soluble polymer in the inert portion of the polymerization mixture is less than about 10%, or when its molecular weight is not high enough, the resulting polymers possess only very low pore volume, if any, [Revillon A. et al., React. Polym., 10, 11 (1989); Seidl J. et al., Chem. Prumysl 13, 100 (1963)]and are not macroporous.
It has to be stressed that there is enormous change in the porous properties of a resulting macroporous polymer when the porogenic agent changes from pure low molecular weight solvent to a solution of linear polymer in the same solvent. A styrene-divinylbenzene copolymer prepared in the presence of toluene as a porogen has pores only in the range of about 3 to 100 nm, while the use of a 12% solution of polystyrene in toluene has been found to produce macropores up to about 2,000 nm.
The critical concentration of the soluble polymer necessary for creating a macroporous structure depends on the amount of crosslinking agent present in the polymerization mixture. When the chemical composition of the soluble polymer used as the porogen is very different from that of the synthesized polymer, the macroporosity begins to occur at both a lower molecular weight and lower concentration, due to the decrease in polymer compatibility. At the beginning of the polymerization, the porogenic polymer chains are dissolved in the monomers. During polymerization a phase separation occurs between the crosslinked copolymer produced and the original soluble polymer, resulting in a shrinking of the coil of the latter. The coiled polymer sterically prevents the network from reaching an ideal structure. The full extent of porosity is realized only when the original polymer is extracted from the matrix by means of one or more low molecular weight solvents. The internal structure of the macroporous polymers contains large agglomerates of globules alternating with large pores having diameters even greater than 1 .mu.m. The porous copolymers previously obtained with the soluble polymer porogen have relatively small specific surface area despite reasonably high porosities that also suggest the presence of large pores.